1. Field
The present invention relates to a lithographic apparatus having a drive system configured to move a substrate support along a trajectory relative to a projection system, and an associated method. The present invention also relates to a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred through a projection system onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Many industrial processes, such as lithography, involve motion of a movable part along a trajectory that is defined by precise positions at specific times, e.g., set-points. Typically, the motion is performed by a closed loop controlled servo system including a motor with an amplifier, mechanics to be actuated (e.g., a slider), a position (and/or a velocity and/or an acceleration) sensor, a feedback and feed-forward controller and a set-point generator. The motor receives the input from the controller that calculates the motor input as a function of the difference between the set-point position and the measured actual position. Feedback control ensures that the actual position will become equal to the desired commanded set-point position.
A method for determining set-point data for such a movable part can be referred to as ‘trajectory planning’ and the resulting set-point data can be referred to as the ‘trajectory’. Typically, motion signals to be applied to one or more actuators, e.g., one or more linear or planar motors, of the movable part are determined from the set-point data of the trajectory, although not necessarily in all circumstances. The motion signals, e.g., set-point signals, are then applied to the actuator to move the movable part to the desired positions.
As an example, motion trajectories are often applied to the substrate and patterning device (e.g. mask) in a step-and-scan lithography apparatus. In a typical one of such apparatus, the substrate surface is exposed in a sequence of field scans. The exposure of each field requires that the substrate and patterning device (e.g. mask) be simultaneously scanned at precisely synchronized, substantially constant velocities. After each field exposure, a substrate stage is stepped from an initial state (i.e., a position and velocity) at the end of a field scan, to a new state (i.e., a new position and typically the same velocity) at the start of the next field scan. Similarly, a patterning device (e.g. mask) stage is also stepped from an initial state at the end of a field scan, to a new state at the start of the next field scan. To maximize processing of substrates per unit time in a lithographic apparatus, i.e., substrate throughput, it is desirable to expose a substrate in the minimum possible time.
A lithographic apparatus includes at least one substrate table or substrate support constructed to hold a substrate. The substrate support is positioned in different, mutually orthogonal directions (hereinafter referred to as x and y directions) by a positioning device that includes respective drive motors for the different, mutually orthogonal x and y directions. An x direction drive motor is used to position the substrate support in a +x or −x direction, and a y direction drive motor is used to position the substrate support in a +y or −y direction. By combining movements in +x, and +y or −y directions, or movements in −x, and +y or −y directions, the substrate support can be positioned anywhere in a (usually horizontal) plane defined by the x and y directions. By positioning the substrate support, the substrate held on the substrate support is positioned accordingly relative to the projection system for performing the stepping and scanning processes referred to above.
In “immersion” type lithographic apparatus, at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that a liquid is located between the projection system and the substrate during exposure. The liquid is captured or enclosed under a cover or hood extending between the projection system and the substrate. A gap between the cover or hood and the substrate is made impassable for the liquid by the use of air knives or other provisions for keeping the liquid between the projection system and the substrate.
In the positioning of a substrate, the x direction and y direction drive motors are controlled independently from each other by using set-point generators for generating set-point data in the x and y directions. The independent control of the x and y direction drive motors incurs that the x direction setpoint generator operates without information relating to the y direction setpoint generator, and vice versa.
Each setpoint generator uses, inter alia, a maximum velocity as a parameter to limit the maximum generated velocity in the corresponding direction. In an immersion type lithographic apparatus, the maximum velocity of the substrate may be essentially determined by the requirement to keep the immersion liquid between the projection system and the substrate, and to prevent the liquid from passing the gap between the cover or hood and the substrate. If the velocity of the cover or hood relative to the substrate would be too high, then liquid would pass the gap, whereby the substrate and its environment would become contaminated with the liquid or components thereof, and liquid would be lost inadvertently from below the cover or hood. This is an undesirable situation.
Indicating the maximum velocity of the substrate support in an immersion type lithographic apparatus as the ‘maximum immersion velocity’, then a problem arises if, for a particular movement, each of the x direction and y direction setpoint generators have their maximum generated velocity set to the maximum immersion velocity. In case of generating a combination of a velocity equal to the maximum immersion velocity in either the x direction or the y direction, and an absolute velocity higher than zero in the other direction, (an absolute value of) a resultant velocity, i.e. a sum velocity vector of the x and y velocities, will in fact exceed (an absolute value of) the maximum immersion velocity. In a worst case scenario, if both in the x direction and in the y direction a velocity is generated to have a value equal to the maximum immersion velocity by the respective setpoint generators, the sum velocity vector will be at 45° relative to the x or y direction, and has an absolute value of square root of two (approximately 1.4) times the maximum immersion velocity. Thus, in fact the maximum immersion velocity may be exceeded considerably, with the adverse consequences described above.